Metallized polyimide film

ABSTRACT

A metallized polyimide film of the present invention comprises a polyimide film  1  which has undergone surface roughening treatment to produce a surface Ra value of 2 to 10 nm, an intermediate layer  2  formed from one, or two or more elements selected from a group consisting of molybdenum, silicon and silicon monoxide, which is formed on top of the surface which has undergone surface roughening treatment with an average thickness of 5 to 50% of the aforementioned Ra value, and a conductive metal layer  4  which is formed on top of the intermediate layer  2 . This construction improves the bonding strength between the polyimide film and the metal layer.

TECHNICAL FIELD

[0001] The present invention relates to a metallized polyimide film inwhich a metal layer of a metal such as copper is formed on the surfaceof a polyimide film, and a production method thereof, and relatesparticularly to a metallized polyimide film used as a TAB tape, aflexible circuit board, or a flexible wiring board or the like.

BACKGROUND ART

[0002] In recent years, the demand for circuit boards using TAB (TapeAutomated Bonding) or FPC (Flexible Print Circuit), which offeradvantages in the trend towards smaller, lighter weight and structurallymore flexible electronic devices, has continued to increase.Conventionally, these types of circuit boards have used a flexibleplastic substrate with a layer of copper foil bonded to this substrateusing an adhesive such as an epoxy based adhesive.

[0003] However, in order to increase the packaging density in electronicdevices, it is desirable to reduce the film thickness of this type ofcircuit board even further, and the structure described above, in whichcopper foil is bonded to a substrate, is unable to fully satisfy thesedemands for even thinner films.

[0004] Furthermore, in the aforementioned circuit boards using anadhesive, (1) the etching solution for the copper foil may penetratethrough to the adhesive layer, and when a bias is then applied underconditions of high temperature and high humidity, copper migration mayoccur, causing a short circuit within the circuit, (2) in order toimprove speed, the impedance must be matched and cross-talk reduced, butthe presence of the adhesive makes this difficult, (3) the dimensionalstability of the adhesive layer is poor, (4) the presence of theadhesive layer makes ultra fine processing of the circuit boarddifficult, making a shift to higher densities problematic, (5) thethermal characteristics of the adhesive layer are inferior to those ofthe plastic substrate material, leading to problems of thermalstability, and making a shift to higher densities problematic, and (6)the presence of the adhesive means that deformation of the product ismore likely to occur.

[0005] In order to resolve these problems, techniques for forming ametallized film without using an adhesive are being investigated. Knownexamples include methods in which thin film formation techniques such asvacuum deposition, sputtering or ion plating are used to form a metallicthin film directly onto a plastic substrate in accordance with a circuitpattern, and a metal plating layer is then deposited on top of thismetallic thin film using electroplating or the like, and methods inwhich a metallic thin film is formed on the surface of a plasticsubstrate, a metal is then deposited on top of this thin film usingelectroplating or the like, and the metal layer is then etched to form acircuit pattern.

[0006] However, in these types of structures, the post processes such asthe circuit pattern formation process and the electroplating processcause a reduction in the bonding strength between the plastic substrateand the metallic thin film, increasing the possibility of the metallicthin film separating from the substrate.

[0007] In order to resolve this problem, a configuration is disclosed inJapanese Unexamined Patent Application, First Publication No. Hei1-133729, in which a film of a zirconium oxide or a silicon oxide isformed on the surface of a polyimide film, and a copper layer is thenformed thereon.

[0008] In Japanese Unexamined Patent Application, First Publication No.Hei 3-274261, a configuration is disclosed in which a film of nickel,chromium, titanium, vanadium, tungsten or molybdenum or the like isformed on the surface of a polyimide film, and a copper layer is thenformed thereon.

[0009] In Japanese Unexamined Patent Application, First Publication No.Hei 5-183012, a configuration is disclosed in which a thin film ofnickel, cobalt, tungsten or molybdenum or the like is formed on thesurface of a polyimide film using electroless plating, and a copperlayer is then formed thereon using electroplating methods.

[0010] In Japanese Unexamined Patent Application, First Publication No.Hei 7-197239, a configuration is disclosed in which a metal such asnickel, chromium, molybdenum or tungsten is deposited on the surface ofa polyimide film using vacuum deposition, and a copper layer is thenformed thereon using electroplating.

[0011] In Japanese Unexamined Patent Application, First Publication No.Hei 8-330695, a configuration is disclosed in which a thin film ofmolybdenum is formed on the surface of a polyimide film usingsputtering, and a copper layer is then formed thereon usingelectroplating.

[0012] However, none of the above methods is able to completely preventthe separation of the copper layer from the polyimide film, and aconfiguration capable of further strengthening the bonding between thecopper layer and the polyimide film has been keenly sought.

[0013] The present invention takes the above circumstances intoconsideration, with an object of providing a metallized polyimide filmwhich is capable of increasing the bonding strength between the metallayer and the polyimide film, as well as a method of producing such ametallized polyimide film.

DISCLOSURE OF INVENTION

[0014] In order to achieve the aforementioned object, a metallizedpolyimide film according to the present invention comprises a polyimidefilm which has undergone surface roughening treatment to produce asurface Ra value of 2 to 10 nm, an intermediate layer formed from one,or two or more films selected from a group consisting of molybdenum,silicon and silicon monoxide, which is formed on top of the surfacewhich has undergone surface roughening treatment with an averagethickness of 5 to 50% of the aforementioned Ra value, and a conductivemetal layer which is formed on top of the intermediate layer. Thethickness of the conductive layer will vary depending on the materialused in the metal layer and the film formation method employed, althoughit is though that typically the thickness will be at least 300angstroms, and at least 1000 angstroms as the film becomes more dense.

[0015] According to this type of metallized polyimide film, a suitabledegree of surface roughening treatment is performed on the surface ofthe polyimide film, and a film of molybdenum, silicon or siliconmonoxide is then formed thereon with a suitable distribution, andconsequently the bonding strength of the metal layer can be improved.

[0016] A method of producing a metallized polyimide film according tothe present invention comprises a step for performing surface rougheningtreatment on the surface of a polyimide film to produce a surface Ravalue of 2 to 10 nm, a step for forming an intermediate layer from one,or two or more films selected from a group consisting of molybdenum,silicon and silicon monoxide, which is formed on top of the polyimidefilm surface which has undergone surface roughening treatment with anaverage thickness of 5 to 50% of the aforementioned Ra value, and a stepfor forming a metal layer of a conductive thickness on top of theintermediate layer.

[0017] The surface roughening treatment described above can utilize atleast one of alkali etching treatment, vacuum plasma treatment, andatmospheric corona treatment.

[0018] A metallized polyimide film of the present invention may also bein the form of a TAB tape, a flexible circuit board, or a flexiblewiring board or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a enlarged cross-sectional view of one example of ametallized polyimide film according to the present invention.

[0020]FIG. 2 through FIG. 4 each represent a schematic illustration ofan intermediate layer bonded to a polyimide film.

BEST MODE FOR CARRYING OUT THE INVENTION

[0021] As follows is a description of preferred examples of a metallizedpolyimide film according to the present invention, with reference to thedrawings.

[0022]FIG. 1 is a schematic illustration showing one example of ametallized polyimide film according to the present invention. Themetallized polyimide film comprises a polyimide film 1 which hasundergone surface roughening treatment, an intermediate layer 2 formedfrom one, or two or more elements selected from a group consisting ofmolybdenum, silicon and silicon monoxide, which is formed on top of thesurface which has undergone surface roughening treatment, and a metallayer 4 which is formed on top of the intermediate layer 2. In thisexample, the intermediate layer 2 and the metal layer 4 are formed onone surface of the polyimide film 1, although the intermediate layer 2and the metal layer 4 may also be formed on both surfaces of thepolyimide film 1, or may be formed so as to generate a certain pattern.

[0023] A first characteristic of the present invention is the fact thatthe surface of the polyimide film 1 onto which the intermediate layer 2and the metal layer 4 are formed has been subjected to prior surfaceroughening treatment, so that the value of the center plane averageroughness (Ra) of the surface is from 2 to 10 nm. Ra values from 2 to 5nm are even more preferred. If the Ra value falls within this range,then the bonding strength between the metal layer 4 and the polyimidefilm 1 can be increased. If the Ra value is too large, then the surfaceof the polyimide film 1 deteriorates, and the bonding strength of themetal layer 4 decreases. In contrast, if the Ra value is too small, thenthe fragile (brittle) layer can not be adequately removed, and thebonding strength of the metal layer 4 decreases.

[0024] The center plane average roughness (Ra) represents the valuedetermined by the formula shown below, wherein a roughness curve isproduced by eliminating low frequency components from a cross-sectionalcurve, the measured surface area S portion is then extracted in thedirection of the center plane of this roughness curve, and the centerplane of this extracted portion is deemed the x axis, the direction ofthe depth magnification ratio is deemed the y axis, and the roughnesscurve is represented by y=f(x,y). Measurements are conducted using acontact mode utilizing the probe of an atomic force microscope, with ameasurement range of 2×2 μm.${Ra} = \left. {\frac{1}{S}{\int_{0}^{Lx}\int_{0}^{Ly}}} \middle| {f\left( {x,y} \right)} \middle| {{x}{y}} \right.$

S=Lx×Ly

[0025] There are no particular restrictions on the type of surfaceroughening treatment used, although preferred techniques include alkalietching treatment, vacuum plasma treatment, or atmospheric coronatreatment, or combinations of two or more of these treatments. Examplesof specific conditions for each type of treatment are described below.

[0026] An alkali etching treatment is a technique in which the polyimidefilm 1 is immersed in an alkali etching solution, thereby dissolving athin layer of the polyimide surface, and removing the fragile layerpresent on the surface of the polyimide film 1. The fragile layer ispresent on the surface of the polyimide film 1 in varying degrees ofthickness, and so following treatment, the surface of the polyimide film1 displays a certain degree of surface roughness. Examples of suitablealkali etching solutions include solutions containing one, or two ormore materials selected from sodium hydroxide, potassium hydroxide,hydrazine hydrate and potassium perchlorate and the like, as well asmixed solutions which also contain ethylenediamine or dimethylamine orthe like. The treatment conditions will vary depending on theconfiguration of the apparatus used, although a concentration of 20 to80 wt %, a solution temperature from 10 to 80° C., and a treatment timeof 30 seconds to 90 minutes are preferred.

[0027] In a vacuum plasma treatment, the polyimide film 1 travelsthrough a vacuum vessel and the surface is irradiated with a plasma,thereby removing the fragile layer on the surface of the polyimide film1. The plasma treatment may use a gas such as argon, oxygen, nitrogen,helium, carbon tetrafluoride, carbon trifluoride or carbon tetrachlorideor the like, or a mixture of two or more of these gases. The energysupply source for providing the energy necessary for plasma generationtypically utilizes direct current (DC), alternating current (AC), radiofrequency (RF), microwaves or an ion beam or the like, and a method forfurther stabilizing the plasma by applying a magnetic field can also beused. The treatment conditions will vary depending on the configurationof the apparatus used, although in the case of a radio frequency powersource, for example, the degree of vacuum within the treatment vesselshould preferably be set to a value from 10⁻² to 100 Pa, the appliedradio frequency set to a value from 0.05 to 1.0 W/cm², and the treatmenttime set to a period from 30 seconds to 20 minutes.

[0028] In an atmospheric corona treatment, the polyimide film 1 travelsthrough a normal atmosphere, and a corona discharge is brought incontact with the surface, thereby removing the fragile layer on thesurface of the polyimide film 1. The treatment conditions will varydepending on the configuration of the apparatus used, although thecurrent through the blade roll which applies the high frequency, highvoltage should preferably be set to a value of 0.5 to 8 A, and thetreatment time set to a period from 30 seconds to 20 minutes.

[0029] There are no particular restrictions on the thickness of thepolyimide film 1, although values from 25 to 125 μm are preferred.

[0030] The material for the polyimide film 1 may utilize any polyimideresin typically used for this type of application, and both BPDA typepolyimide resins and PMDA type polyimide resins are suitable. Generally,polyimide films using BPDA (biphenyltetracarboxylic acid) as a rawmaterial (such as the commercial product “Upilex” manufactured by UbeIndustries) offer superior dimensional stability under heat and moistureabsorption, as well as excellent rigidity, and are mainly used for TABapplications, although the bonding strength of these polyimide filmswith metallic thin films is low. In contrast, polyimide films using PMDA(pyromellitic dianhydride) as a raw material (such as the commercialproduct “Kapton” manufactured by DuPont-Toray Co., Ltd., or thecommercial product “Apical” manufactured by Kaneka Corporation) offergood bonding strength with metallic thin films.

[0031] The polyimide film 1 may be a single layer, although a laminatedfilm comprising a plurality of different polyimide resins laminatedtogether is also acceptable. The same effects can be achieved whetherthe surface of the polyimide film 1 contacting the intermediate layer 2is a BPDA type polyimide or a PMDA type polyimide.

[0032] A second characteristic of the present invention is the fact thatthe average thickness of the intermediate layer 2 is from 5 to 50% ofthe Ra value of the polyimide film 1. Average thickness values of theintermediate layer 2 of 30 to 50% are even more preferred. If thethickness falls within this range, then the production costs are not toohigh, and the improvement in the bonding strength with the metal layer 4is excellent. If the intermediate layer 2 is too thick, then when thewiring pattern is formed by etching, the quality of the etchingdeteriorates. If the intermediate layer 2 is too thin, then the filmthickness of the intermediate layer 2 begins to lose uniformity, andcontrol of the film thickness becomes problematic, and the bondingstrength of the metal layer 4 to the polyimide film 1 also decreases.

[0033] The intermediate layer 2 may be bonded uniformly to the surfaceof the polyimide film 1, as shown in FIG. 2, although an islands typeintermediate layer 2 will also exhibit the effects of the presentinvention. In other words, the intermediate layer 2 may be concentratedon the convex portions 1A on the surface of the polyimide film 1 asshown in FIG. 4, or concentrated within the concave portions 1B of thepolyimide film 1 as shown in FIG. 3. In these types of cases, where thebonding occurs in a non-uniform manner, the average thickness of theintermediate layer 2 refers to the average thickness calculated bydividing the quantity of the intermediate layer 2 by the bonding regionsurface area of the polyimide film 1.

[0034] The material for the intermediate layer 2 may be one, or two ormore materials selected from a group consisting of molybdenum, siliconand silicon monoxide, although according to experiments by theinventors, of these materials, molybdenum displays a particularly largebonding strength, and is also superior in terms of maintaining a highbonding strength following a variety of durability tests.

[0035] The material for the metal layer 4 may be one, or two or moremetals selected from copper, copper alloy, aluminum, aluminum alloy,silver, gold or platinum and the like, although pure copper, or copperalloys containing nickel, zinc or iron and the like are particularlypreferred. The thickness of the metal layer 4 may be any thicknesssufficient to achieve conductivity. The thickness required to achieveconductivity will vary depending on the material used in the metal layerand the method used to form the layer, although it is though thattypically the thickness will be at least 300 angstroms, and at least1000 angstroms as the film becomes more dense. A thickness of more than2000 Å is also possible. If the metal layer 4 is too thick, then thecost will become overly expensive, whereas if the layer is too thin,then problems such as burn out are more likely to occur during theplating process.

[0036] When forming the metal layer 4, a thin film formation techniquesuch as vacuum deposition, sputtering or ion plating can be used tosimply form a film of metal on the polyimide film 1 coated with theintermediate layer 2, or alternatively, a thin film can first be formed,and a metal plating layer can then be deposited on top of this filmusing either an electroplating method or an electroless plating method.

[0037] According to a metallized polyimide film described above and amethod of producing such a film, the bonding strength between thepolyimide film 1 and the metal layer 4 can be increased, and thissuperior bonding strength can be maintained following durability tests.Furthermore, a large bonding strength can be achieved regardless ofwhether the interface contacting the intermediate layer 2 is a BPDA typepolyimide or a PMDA type polyimide.

EXAMPLES

[0038] Next, the effects of the present invention are substantiated bypresenting a series of examples.

Example 1

[0039] The commercial product “Upilex S” (thickness 50 μm) manufacturedby Ube Industries was used as a BPDA type polyimide film material, andthis film material was subjected to plasma treatment using argon gas,under conditions including a radio frequency of 0.4 W/cm² and atreatment time of 5 minutes. The Ra value for the film materialfollowing this treatment was 2.73 nm.

[0040] This surface treated film was then set inside a sputteringapparatus, and an intermediate layer and a metal layer were formed onthe plasma treated surface under the conditions described below.

[0041] Intermediate layer material: molybdenum

[0042] Film formation conditions: argon gas, high frequency output 200 W

[0043] Film thickness: 1 nm=36.6% of the Ra value (2.73 nm)

[0044] Metal layer material: copper

[0045] Film formation conditions: argon gas, high frequency output 200 W

[0046] Film thickness: 300 nm

[0047] A copper sulfate bath was then used to form a 20 μm electroplatedlayer of copper on the metal surface of the metallic thin film, therebyobtaining a metallized polyimide film of the example 1.

Example 2

[0048] The commercial product “Kapton II” (thickness 50 μm) manufacturedby DuPont-Toray Co., Ltd. was used as a PMDA type polyimide filmmaterial, and by performing treatment in the same manner as the example1, a metallized polyimide film of the example 2 was obtained. The Ravalue following surface treatment was 2.93 nm (and consequently thethickness of the intermediate layer was 34.1% of the Ra value).

Comparative Example 1

[0049] The commercial product “Upilex S” (thickness 50 μm) manufacturedby Ube Industries was used as a BPDA type polyimide film material. TheRa value for this untreated film material was 1.38 nm. This filmmaterial was set inside a sputtering apparatus, and a metal layer wasformed under the conditions described below.

[0050] Metal layer material: copper

[0051] Film formation conditions: argon gas, high frequency output 200 W

[0052] Film thickness: 300 nm

[0053] A copper sulfate bath was then used to form a 20 μm electroplatedlayer of copper on the metal surface of the metallic thin film, therebyobtaining a metallized polyimide film of the comparative example 1.

Comparative Example 2

[0054] The commercial product “Kapton II” (thickness 50 μm) manufacturedby DuPont-Toray Co., Ltd. was used as a PMDA type polyimide filmmaterial. The Ra value for this untreated film material was 1.54 nm.This film material was treated in the same manner as the comparativeexample 1, thereby obtaining a metallized polyimide film of thecomparative example 2.

Comparative Example 3

[0055] The commercial product “Upilex S” (thickness 50 μm) manufacturedby Ube Industries was used as a BPDA type polyimide film material. TheRa value for this untreated film material was 1.39 nm. This filmmaterial was set inside a sputtering apparatus, and an intermediatelayer and a metal layer were formed under the conditions describedbelow.

[0056] Intermediate layer material: molybdenum

[0057] Film formation conditions: argon gas, high frequency output 200 W

[0058] Film thickness: 1 nm

[0059] Metal layer material: copper

[0060] Film formation conditions: argon gas, high frequency output 200 W

[0061] Film thickness: 300 nm

[0062] A copper sulfate bath was then used to form a 20 μm electroplatedlayer of copper on the metal surface of the metallic thin film, therebyobtaining a metallized polyimide film of the comparative example 3.

Comparative Example 4

[0063] The commercial product “Kapton II” (thickness 50 μm) manufacturedby DuPont-Toray Co., Ltd. was used as a PMDA type polyimide filmmaterial. This film material was treated in the same manner as thecomparative example 3, thereby obtaining a metallized polyimide film ofthe comparative example 4. The Ra value for the untreated film materialwas 1.57 nm.

Comparative Experiments

[0064] A strip specimen of width 10 mm×length 150 mm was cut from eachof the metallized polyimide films of the examples 1, 2 and thecomparative examples 1 to 4, and the bonding strength between the filmmaterial and the metallic thin film was measured using the methodaccording to IPC-TM-650 (U.S. printed circuit industrial standard testmethod). In this test method, the polyimide film side of anaforementioned strip specimen is bonded and fixed to the externalsurface of a 6 inch diameter drum so as to extend around thecircumference of the drum, and one end of the metallic film is thenpulled away and separated from the polyimide film using a jig moving at50 mm/minute, and the load required to achieve this separation ismeasured. The results are shown in Table 1.

[0065] Furthermore, a pressure cooker test (PCT) was also performed oneach strip specimen, and the same bonding strength test described abovewas repeated on the metallized polyimide film following PCT, giving anindication of the bonding strength following a durability test. Theresults are shown in Table 1. The conditions for the PCT included atemperature of 120° C., humidity of 100%, 2 atmospheres, and 48 hours.TABLE 1 Immediately following film formation Following PCT test Example1 1220 g/cm 1150 g/cm Example 2 1510 g/cm 1430 g/cm Comparative Example1  182 g/cm  50 g/cm Comparative Example 2  312 g/cm  150 g/cmComparative Example 3  310 g/cm  200 g/cm Comparative Example 4 1020g/cm  450 g/cm

INDUSTRIAL APPLICABILITY

[0066] According to a metallized polyimide film of the presentinvention, a suitable surface roughening treatment is performed on thesurface of the polyimide film, and a film of molybdenum, silicon orsilicon monoxide is then formed thereon with a suitable distribution,and consequently the bonding strength of a metal layer can be increased.Accordingly, the present invention is ideally suited to TAB tapes,flexible circuit boards, and flexible wiring boards.

1. A metallized polyimide film comprising a polyimide film which hasundergone surface roughening treatment to produce a surface Ra value of2 to 10 nm, an intermediate layer formed from one, or two or moreelements selected from a group consisting of molybdenum, silicon andsilicon monoxide, which is formed on top of a surface which hasundergone said surface roughening treatment, with an average thicknessof 5 to 50% of said Ra value, and a conductive metal layer which isformed on top of said intermediate layer.
 2. A method of producing ametallized polyimide film comprising a step for performing surfaceroughening treatment on a surface of a polyimide film to produce asurface Ra value of 2 to 10 nm, a step for forming an intermediate layerfrom one, or two or more elements selected from a group consisting ofmolybdenum, silicon and silicon monoxide, which is formed on top of saidsurface of said polyimide film which has undergone said surfaceroughening treatment, with an average thickness of 5 to 50% of said Ravalue, and a step for forming a conductive metal layer on top of saidintermediate layer.
 3. A method of producing a metallized polyimide filmaccording to claim 2, wherein said surface roughening treatment is atleast one treatment selected from alkali etching treatment, vacuumplasma treatment and atmospheric corona treatment.